The present invention refers to a method of determining the spatial relationship of an eye of a person with respect to a camera device which provides images of the eye.
More particularly, the invention refers to a model based technique for eye tracking using a single camera, in particular a single digital camera. Eye tracking applications are numerous and range from gaze tracking, i.e. analyzing which object a person is looking at for marketing studies, research in neuroscience or military applications to eye tracking in medical applications.
The present invention has potential in a number of eye tracking applications, with the largest relevance in the field of medical eye tracking during corneal ablation surgery.
The term “eye tracking” usually refers to the repeated or even continuous estimation of eye movements of a person. However, in the context of the present invention, this term may also be used for a single determination of the spatial relationship of the person's eye with respect to the camera device based on the current eye image.
As in prior art methods of determining the spatial relationship of a person's eye with respect to a camera device, the present invention is based on the assumption that the eye has six degrees of freedom to move in three-dimensional space, namely three translations defining the eye's position as well as three rotations defining the eye's orientation, as is illustrated in FIG. 1.
Some eye tracking systems use head mounted cameras or a camera attached to a head rest or a chin rest. In this special case all eye movements with respect to the camera can be interpreted as rotations and after calibration the line of sight can be recovered by detecting the pupil in the camera image. These systems are intrusive and uncomfortable though and require the use of a headrest, a chin rest or a helmet.
Standard non-intrusive techniques detect the pupil in the camera image together with corneal reflexes (so called 1st purkinje images) caused by illumination sources at a defined position relative to the camera. When using at least two illumination sources it is possible to recover five degrees of freedom of the eye, three translations and two rotations. Alternatively some approaches not only use corneal reflections but also reflections from the eye's lense. All these reflections of refractive surfaces are called purkinje images. It is not possible to recover the rotation around the symmetry axis of the eye using purkinje images. These techniques are used for gaze tracking in different applications and are usually referred to as purkinje approaches.
Other non-intrusive techniques for eye tracking using one digital camera simply detect the pupil in the 2D camera image. With this information it is not possible to distinguish between eye rotation and eye translation. Systems like this are often employed in ophthalmic treatment lasers for corneal ablation. These systems track the pupil but the ablation is performed on the cornea which is located appr. 3 mm in front of the pupil. Systematic ablation errors do occur if large eye rotations happen during surgery since they are solely interpreted as translations (or vice versa). The detection of corneal reflections to be able to distinguish between eye rotation and eye translation is not possible in this application, because in the standard procedure for corneal ablation (LASIK) a thin layer of the cornea having a thickness of approximately 180 microns is cut off. The cornea surface is not smooth anymore and corneal reflections can not be localised accurately. This is illustrated in FIGS. 2a and 2b showing purkinje images of a person's eye before and after the thin cornea layer has been cut of, respectively. In FIG. 2a, the cornea of the person is intact, so that the corneal reflexes of two illumination sources located at a defined position in space can be accurately detected. The cornea surface in this case acts more or less like a mirror. In FIG. 2b, however, the same eye is shown after a flap cut. The cornea surface is not smooth anymore and corneal reflexes can no longer be detected. The standard purkinje approach to determine the position of the eye as well as two among the three rotation coordinates can therefore no longer be used.
It is therefore an object of the invention to provide a method of determining the spatial relationship of an eye of a person with respect to a camera device which does not rely on corneal reflections, which is less intrusive and uncomfortable than the above-discussed prior art tracking methods, and which allows to recover all eye movements in six degrees of freedom.